Indeed, an encrypted root fs and swap file will decrease performance as I pointed out at the end of the article. (i edited the article now to explain the performance cost at the beginning)
It would be ideal to have closer to 8GB RAM for the node if the disk is fully encrypted, that way the challenge is not caching to disk instead of using RAM.
There is also the option of testing AES encryption (as opposed to serpent) -- AES is usually hardware accelerated as most CPUs these days have AES instruction sets. A KVM VPS/Virtual Machine should be able to use the host CPU acceleration, but only if the VPS/Virtual Machine is using host-passthrough or similar config for the guest CPU (see https://forum.proxmox.com/threads/aes-ni-extension-on-guest-cpu.12898/)
running cat /proc/cpuinfo | grep aes on the Vultr VPS shows aes flag is on the virtual CPU, so aes may be a bit faster than serpent, but it still may fail the challenge with just 1GB RAM